docs: def _populate_validated_data_with_sub_field_data(self, validated_data):
          """
          Move field data nested in `ModelSubSerializer` fields back into the
          overall validated data dict.
          """
          for fieldname, field in self.get_fields().items():
              if isinstance(field, ModelSubSerializer):
                  field_data = validated_data.pop(fieldname, None)
                  if field_data:
                      validated_data.update(field_data)